KH-4 Corona (Agena-D based)

KH-4 18 [USAF]

The KH-4 (Keyhole-4) was fourth version in the Corona-program to build an optical reconnaissance satellite. Only the first two satellites were launched under the name Discoverer, before this designation was dropped.

The KH-4 payload introduced a second camera to provide stereoscopic imaging. The Corona-M or Mural camera consisted of two C''' (Corona triple-prime) cameras inherited from KH-3. The cameras were mounted with 30 degrees separation angle. The two cameras are referred to as fore (for forward looking) and aft (for rear looking). KH-4 also carried a 4 cm index camera, with a focal length of 38 mm, a ground resolution of 162 m, and frame coverage of 308 km × 308 km. The Film was returned by a single General Electric return vehicle (SRV, Satellite Return Vehicle). The Mural-camera was built by Itek Corporation and had a ground resolution of 7.5 m.

The satellite was based on the Agena-D upper stage, which provided attitude control in orbit. The film return capsule carried its own small solid-fuel retro motor to deorbit at the end of the mission with mid-air recovery of the returning capsule by a specially equipped aircraft.

Earlier KH-4 missions were based on the Agena-B stage.

Some missions featured piggy-back payloads, which remained connected to the Agena-D stage

• KH-4 7 carried the GRD-N-Payload No. 4
• KH-4 11 carried the RM 3 radiometer payload for background measurements for the MIDAS program
• KH-4 13 carried the RM 4 radiometer payload (identical to RM 3) for background measurements for the MIDAS program
• KH-4 24 carried the RM 5 radiometer payload for background measurements for the MIDAS program

Two missions carried small satellites into orbit.